1. Field of the Invention
The present invention relates to an improvement in a semiconductor laser of planar type.
2. Description of the Prior Art
Semiconductor laser has advantages of smallness of bulk, high efficiency and direct modulation by means of its current, and therefore has a bright future as light sources for optical communication, optical data processing. Laser for such use necessitates characteristics of stable fundamental transverse mode lasing, low threshold current, high output of light and high reliability.
The conventional laser which has a structure of simple gain guiding has a difficulty in maintaining a transverse mode for a wide range of current, and therefore is liable to occurrence of undesirable mode conversion or a generation of higher modes. As a result of these, the light-current characteristic is likely to have a kink of characteristic curve or the device is likely to have a multiple longitudinal mode oscillation.
FIG. 1 shows a conventional semiconductor stripe laser of the planar type. The laser of FIG. 1 has a double-hetero structure which has on
______________________________________ a substrate 1 of n-GaAs a first clad layer 2 of n-GaAlAs, an active layer 3 of non-doped GaAlAs, a second clad layer 4 of p-GaAlAs and an isolation layer 5 of n-GaAlAs, which forms a p-n isolation junction between it and the underlying p-type second clad layer 4 and has a -stripe-shaped current p-type injection region 6 of diffused region formed by diffusing an acceptor such as Zn, in a manner to penetrate it and diffuse into the midway of the second clad layer. ______________________________________
Numeral 7 and 8 designate p-side and n-side electrode ohmicly contacting the current injection region 6 and the substrate 1, respectively.
In such planar stripe laser, by nearing the diffusion front of the current injection region 6 to the active layer 3, the spread of current path is suppressed to some extent. However, minimizing of the width of the current path 6, which is mainly determined by the width of the diffused region 6, is limited. And furthermore, when a very narrow diffused region is used, an internal series resistance of the laser becomes large. On the other hand, decreasing of threshold current requires limiting of injected current in a very narrowly limited region, and the narrowness of width of the spreading current in the active layer mainly determines stability of the transverse mode oscillation. Therefore, the width of the spread of the injected current in the active layer, or effective width of the current path in the active layer should be narrowed as much as possible in order to obtain good semiconductor laser.